The decimal notation of a real number is often present in everyday situations. Students encounter decimal notation for the first time in the fifth grade and in later years they get to know some properties of the decimal notation of real numbers. The decimal notation of a real number is described in the first chapter. It is proved that every real number has at least one notation in decimal form and an algorithm for determining the decimal notation is given as well. The second chapter contains the decimal notation of natural numbers and integers and a proof that every natural number has a decimal representation. In the third chapter, the decimal notation of rational numbers is elaborated. The characterization of rational numbers is presented and proven by observing the period of the decimal notation. The properties of finiteness and periodicity of the decimal notation of rational numbers are also stated and proved and supported by examples. The length of the period and the preperiod was briefly considered. The fourth chapter deals with the decimal notation of irrational numbers, so the characterization of irrational numbers is shown by observing the decimal notation and the irrationality of Champernowne’s constant is proven. In the fifth chapter, selected interesting facts about the decimal notation of real numbers are given, such as cyclic permutations of decimals and Midy’s theorem. The sixth, also the last chapter, deals with the decimal notation of real numbers in mathematics lessons. Prescribed learning outcomes are briefly described and selected examples of tasks used in mathematics teaching in primary and secondary education are solved.